1. Field of the Invention
The present invention relates to a method of and an apparatus for jitter correction of a polygon mirror in order to reduce dot misregistration in scanning lines caused by rotational errors of the polygon mirror in an image recording apparatus such as a flat scanning type facsimile or electronic process scanner, and for performing image recording by scanning recording beams modulated in response to variable density levels (including black and white) of respective dots in synchronization with dot recording clock signals by a polygon mirror on a photosensitive material such as a photographic paper or a film.
2. Description of the Prior Art
In a flat scanning type image recording apparatus employing laser beams as recording beams, a polygon mirror is generally used as a scanning deflector for the laser beams. However, the polygon mirror inevitably creates rotational errors whereby scanning lengths vary with respective reflecting mirror surfaces of the polygon mirror to cause dot misregistration in scanning lines, as shown in FIG. 1. Although the start positions of respective scanning lines are substantially registered or even in the figure, dot misregistration or unevenness by a distance corresponding to 1/2 dot occurs as compared to a correct scanning length L in the end position of the second scanning line. Such dot misregistration deteriorates the picture quality of the recorded image. Particularly an electronic process scanner for duplicating halftone images is adapted to record regular halftone images (halftone dot patterns), and hence misregistration of even 1/2 dot significantly deteriorates the picture quality.
In general, dot misregistration in scanning lines caused by rotational errors of a polygon mirror has been corrected as follows:
A first method employs grating (linear encoding). In this method, a grating laser is assembled in the same optical system with a recording laser. These laser beams are deflected by a polygon mirror that performs exposure recording/scanning by the recording laser beam while grate scanning the grating laser beam to obtain pulse signals indicating positions of the recording laser beam. Dot misregistration in the respective scanning lines is eliminated by creating dot recording clock signals synchronous with the pulse signals through a synchronous control circuit such as a PLL circuit. Although dot misregistration is eliminated by this method, the optical system is complicated since the grating laser beam is assembled in the optical system. Adjustment is complicated and the PLL circuit is increased in cost when high resolution and high frequency are required.
In a second method, a crystal oscillator is combined with a start sensor. This method employs a polygon mirror of high accuracy as a deflector with a reference clock signal generated by the crystal oscillator creating a plurality of clock signals by delaying the reference clock signal by appropriate times, synchronizing the clock signal with a recording laser beam and start sensor in order to process the same as a dot recording clock signal in the corresponding scanning line, and thereby to register the start positions of respective scanning lines. A main scanning synchronous system in this method is simple in structure and hence the same can be relatively easily adjusted, and manufactured at a relatively low cost. In this method, however, the polygon mirror is inherently inferior in accuracy to the crystal oscillator and the start sensor. Thus, although the respective scanning lines substantially coincide in timing with each other without dot misregistration immediately after starting, dot misregistration is inevitably caused by slight rotational errors of the polygon mirror in the vicinity of the end positions, causing terminating ends of the scanning lines to be irregularized. Even when a polygon mirror of higher accuracy is attained in order to prevent this irregularization, such attempts are restricted by both cost and technical limitations.
A third method is adapted to modulate the frequency of a dot recording clock signal in response to rotational errors. This method analogously obtains voltage responsive to the rotational speed of the polygon mirror to perform voltage-frequency (V-F) conversion of the same in order to generate the dot recording clock signal, while digitally generating a reference clock signal to frequency-divide the reference clock signal in a frequency dividing ratio responsive to the rotational speed of the polygon mirror, thereby generating the dot recording clock signal. However, this method requires voltage ratio values of high accuracy, and selection. When the wanted frequency is finely changed in the latter method adjustment of the frequency is difficult.